2o6 ELIOT BLACKW ELDER 



noteworthy changes of level, that it is probably safe to refer the 

 peneplain to the time between the Middle Miocene and these early 

 Quaternary disturbances. 



The complete planation of soft rocks may be accomplished in 

 a geologically short time, but the reduction of very resistant grani- 

 toid rocks to a peneplain must require a vastly longer time. Plana- 

 tion of either kind of rocks by streams demands a constancy of 

 conditions which is possible only when the lithosphere is in a state 

 of rest. Inasmuch as such quiet is rarely local only, and if long 

 continued would permit the planation of very large areas, it is 

 worth while to inquire if peneplains were made in adjacent regions, 

 and if so at what time geologically. 



Within the past decade several geologists' who have independ- 

 ently recognized peneplains in the Rocky Mountains have referred 

 them to late Tertiary time or specifically to the Pliocene. In 

 several other districts somewhat more remote, — the Cascade Range 

 of Washington,^ the Sierra Nevada in California,^ and the Grand 

 Canyon of Arizona,"* — peneplains have been reported and their ages 

 determined as approximately Pliocene. The summit peneplain 

 of central Idaho is assigned by Umpleby^ to the Eocene, but in a 

 review^ I have suggested that the opinion is not well founded. 



In brief, the evidence from all points of view here considered, 

 although it does not establish the age of the peneplain, does strongly 



I W. W. Atwood and K. F. Mather, Jour. GeoL, XX (1912), 407; S. H. Ball, U.S. 

 Geol. Survey, Prof. Paper 63, 1908, 32; E. Blackwelder, "Cenozoic History of the 

 Laramie Region," Jour. Geol., XVII (1909), 437; J. L. Rich, "Physiography of the 

 Bishop Conglomerate, Southwestern Wyoming," Jour. Geol., XVIII (1910), 613 

 (age given as post-Oligocene, probably Miocene); L. G. Westgate and E. B. Branson, 

 "Cenozoic History of the Wind River Mountains," Jour. Geol., XXI (1913), 144. 



^'B. Willis and G. O. Smith, "A Contribution to the Geology of the Cascade 

 Mountains," U.S. Geol. Survey, Prof. Paper ig, 1903, 70. 



3 F. L. Ransome, "The Great Valley of CaUfornia," Univ. of Calif., Bull. Dept. 

 of Geol., 1896, I, 371-428. 



^ H. H. Robinson, "A New Erosion Cycle in the Grand Canyon District, Arizona," 

 Jour. Geol., XVIII (1910), 742-63. 



s J. B. Umpleby, "The Old Erosion Surface in Idaho," Jour. Geol., XX (1912), 

 144- 



^ E. Blackwelder, "The Old Erosion Surface in Idaho: A Criticism." Jour. 

 Geol., XX (191 2), 410-14. 



